The present disclosure is related to the structure of a light emitting device such as a light emitting diode or laser diode, and more specifically to such devices formed over group III-nitride substrates such as semi-polar GaN(1122) and non-polar GaN(1010) substrates. A semi-polar surface orientation of a wurtzite material may be defined as an orientation (h, k, −(h+k), l) with h or k not equal to zero and l not equal to zero.
Producing light emitting structures such as light emitting diodes (LEDs) and semiconductor lasers on c-axis substrates is well known. Al2O3(0001) (c-plane sapphire) is a common c-plane substrate used today. Due to the differences in lattice constants between the sapphire and material grown thereover, such as gallium nitride (GaN), vertical defects such as dislocations and stacking faults arise, which result in crystalline defects in the material grown over the substrate. Lateral overgrowth is one technique used to reduce the vertical dislocation defects.
While c-plane oriented substrates have been the most widely used substrates to date, other orientations such as semi-polar and m-plane orientations are becoming increasingly important. For example, bulk semi-polar GaN substrates are highly desired for indium-based light emitters such as light-emitting diodes (LEDs) and laser diodes (LDs), in order to reduce internal electric fields that impair the efficiency of the light emission process on conventional c-axis oriented nitride devices. However, such bulk substrates are not yet widely available and are limited to small sizes. As an alternative to bulk semi-polar GaN substrates, semi-polar GaN templates have been grown on large area sapphire substrates by conventional means such as Hydride Vapor Phase Epitaxy (HVPE). However, the defect density in such template layers is on the order of 1010 cm−2, unless defect reduction techniques are applied.
While lateral overgrowth is an effective technique for c-plane oriented substrates, it is not optimized for materials in which the c-axis is tilted with respect to the surface normal, such as any semi-polar oriented GaN, in which a significant portion of defects extend across the GaN layer at an angle corresponding to the tilt of the basal plane GaN(0001). One difficulty observed is that since the lattice defects in a semi-polar template layer (or equivalently, a semi-polar substrate) run diagonally, e.g., at a given angle between 0 and 90 degrees relative to the plane of the growth surface, the effectiveness of a mask at limiting extension (or propagation) of the defects into the growth layer is reduced. To compound this problem, certain substrate orientations present defects in multiple different planes (e.g., perpendicular to the growth surface as well as angled relative to that plane). One technique used to address problems presented when using substrates with other than c-axis orientation is epitaxial lateral overgrowth (ELOG), and one variation on the ELOG process, referred to herein as windowed ELOG, is disclosed and discussed in detail in U.S. patent application Ser. No. 12/562,675, which is incorporated by reference herein and to which priority is hereby claimed. According to the windowed ELOG technique, a patterned mask with “window” openings is formed over the semi-polar layer. The windows have a vertical height at least equal to the product of the window width times the cotangent of the angle between the surface normal and the c-axis direction for the semi-polar layer. These windows effectively provide significant suppression of all diagonally running defects during growth of layers over the mask.
However, useful light emitting devices formed over relatively large semi-polar substrates has yet to be demonstrated. For example, typical light emitting structures such as laser diodes utilize a gallium nitride (GaN) template layer with a AlxGa1-xN/GaN short-period super-lattice (SPSL) lower (and upper) cladding layer. Such cladding layers are necessary to confine the optical wave to a region of high optical amplification and low loss due to absorption and scattering. The SPSL addresses the problem of strain-induced cracking that arises with thick AlGaN layers (of equal average composition).
However, the limited thickness, and limited Al-content in particular, of the lower cladding layer may lead to significant leakage of the optical mode into the underlying layer structure. In addition, when formed over the window ELOG base, a corrugated interface is produced as a result of the masking procedure and subsequent overgrowth. Such a corrugated interface results in highly undesired scattering losses. Perhaps most fundamentally, functional devices formed over the window ELOG base have not yet been demonstrated.